In the following text I bound together the posts I wrote sparsingly. I also corrected some obvious flaws. The purpose is to help in giving possible ways to simplify and to optimize a next generation of 3.x rules set. I'm not a game designer and I'm not considering I'm good at that. However, I have a very long experience in playing and Dming and thus I imagine I have a pretty large know how in using pen and paper RPG. It is that one I use to gather the following thoughts.
If you are interrested to react, please do it on the Paizo genuine PFRPG thread you followed up to here. Enjoy !

Summary (current)

Initiative Check

Actions

Tracking & Loging of the actions

Armor Class evolution

The new AC (nAC) definition

The new DR (nDR) definition

Conversion and Compatibility

What about the 3.5 Ed DR ?

A - Initiative Check
Wellll ! I have to start by something and I propose Initiative check.
This is by the rules a dexterity check. It means we have to roll the dices once at the beginning of each combat sequence. So 6 players and say 4 monsters give 10 rolls just to determine an order of actions. Plus, sadly a bad roll will put the quicksilver hero at the last of the pile. Also, it demands to each player and the DM to find a modifier score on a sheet and to proceed in writing the plot chart for each new fight of the session. Not often a great moment of Role Play IMHO.
So:
We calculate the Initiative once using 10 + the current defined and applicable modifiers (DEX, feats, armor penalty to check, racial bonus ?). The chart is done once for all for the players. If two or more Initiative are the same your left guy got the turn – DM is the most left guy of the table. Your never need to change the Initiative score during a sequence as 3.5 suggests – I will explain how in the chapter about Actions.

B - ActionsAbout Acting in combat…
Three different groups are existing for the actions: Proactive, Reactive or Reflex.
Basic Idea is:
Proactive use full bonuses and full speed
Reactive use half bonuses but full speed
Reflexes is indeed when you need to use a Saving Throw of any kind

Rule:
Normally, you are only allowed to take two actions in a round starting at your turn but not more than one Reactive. Before your action turn in a round you cannot have any action but Reflexes one. After your action turn you can only have one Reactive action if available. If you don’t use your reactive action before your next turn it is lost. You can have as many as required Reflexes action in a round. Who play first, ProActive or Reactive ?... or an other story of the chicken and the egg... The rule is that the Reactive action is usually played first, then the Proactive. Example: The Orc, few meters ahead, is playing after Mounhok the ranger who kept one Reactive action after one Proactive he had earlier in sequence. The Monster choose to advance in the direction of the ranger and the DM declares that intention. If the Mounhok's player wants to React he must declare it just now. He has two main options (for this example): or he tries to move away; or he stands and fight. If Mounhok choose the first option, then the orc must maintain his action of moving - however, it is not mandatory to do a full move - but he can reconsider its second action, even in the way of keeping it as a Reactive one. If applicable, he can also declare now a charge which is a special Proactive action which allows a free full move before an attack action. He can also stop his movement after two or three steps and hurl his spear on Mounhok the Coward

Only few situations and effects are able to change the above rule:

Some conditions or spell effect can forbid Proactive or Reactive or both and even Reflexes action,

Some spells effects or feat can allow you one more action or one more Reactive action,

Some actions can never be done twice in a round even if the acting rule would allow it.

Charge, spell and spell like abilities use

Non permanent magic devices use

Some of the Supernatural or Extraordinary effect, feats or abilities

Few actions are considered free and thus don’t count for the limited number of actions. They are the one given by the 3.5 OGL chart of free actions. The use of some feat or of a sounded procedure allows changing an action in a free action (quick reload, belt for wands and so on…).

The system to combine one action of moving with one other action is quite simple: you can always insert one action at every step of your action of moving and after you can try to finish your move. In some case your opponent can stop you through a sounded Reactive action (if he has one). Usually a Reactive action of attack cannot stop a ProActive move. On the reverse way, a ProActive action of attack don't usually stop a Reactive action of Moving.

Now if you consider the above rule with care I think it’s possible to demonstrate it is no more needed to implement the attack of opportunity system. Without it I guess you get however a cinematic system of combat which will pose a threat against the attempt of every action if in the (extended) range of the opponent. Please note that it is also much more instinctive to rule the active and reactive phase IMHO.
One of the question could be why do you halve the bonuses in reactive mode of acting, answer is quite simple: I want the players active and also when Reactive I think you are losing a part of your efficiency in the delay / analysis / decision of action process.

Just below I give the list of the actions like the srd.org give it and I comment and suggest some equivalence:

Attack is a Pro or Rea action You cannot use it more than once in a round as Pro action in this regular rule without having a special ability, feat or multiple attacks – side note:iterative attacks rule will be changed as a feat related thing explained in a further chapter. You can never use Attack more than once in a round as a Rea action but if you have multiples attacks you can use them all. Note again that in Rea mode the bonuses are halved.

Melee Attacks count as 1 action / Pro or Rea

Ranged Attacks count as 1 action / Pro or Rea

Fighting Defensively as a Standard Action count as 1 action / Pro only

Total Defense count as 1 action / Pro or Rea

Charge count as 1 action / Pro only, a charge is always after an action of movement. It allows a free move before your actual attack but no more after, even a Reactive one later in the sequence.

Cast a Spell count as x actions / Pro or Rea, some spells can take more than 1 action

Concentrating to Maintain a Spell count as 1 action / Pro only

Casting on the Defensive count as 1 action / Pro or Rea

Touch Spells in Combat count as 1 action / Pro or Rea

Dismiss a Spell count as 1 action / Pro or Rea

Activate Magic Item count as x actions / Pro or Rea, some activation can take more than 1 action

Spell Completion Items count as 1 or + action / Pro or Rea, but always after the opponent action if any.

Spell Trigger, Command Word count as 1 action / Pro or Rea

Use Special Ability count as 1 or + action / Pro or Rea

Spell-Like Abilities count as 1 or + action / Pro or Rea

Supernatural Abilities count as 1 or + action / Pro or Rea

Extraordinary Abilities count as 1 or + action / Pro or Rea

Use Feat count as 1 action / Pro or Rea

Use Skill count as 1 or + action / Pro or Rea, some skill can take more than one round.

Start/Complete Full-Round Action is no more useful

Move Actions count as 1 or + action / Pro or Rea

Move count as 1 or + action / Pro or Rea

Draw or Sheathe a Weapon count as 1 or - action / Pro or Rea, but can be free with appropriate feat.

Ready or Loose a Shield count as 0 Free

Manipulate an Item count as 1 or + action / Pro or Rea, some manipulations can take more than 1 action

Direct or Redirect a Spell count as 1 action / Pro or Rea

Stand Up count as 1 action / Pro or Rea

Mount/Dismount a Steed count as 2 or - action / Pro or Rea, at least 2 actions are needed if not special skill check.

Run count as 2 or + action / Pro or Rea, a run is 1 move (at least) + 1 move and cannot start in a case in touch with an opponent (if not it’s a withdraw).

Move 5 Feet through Difficult Terrain is no more useful, reduce only the speed.

Drop an Item count as 0 FreeDrop Prone count as 0 Free Speak count as 0 Free(if one short sentence only)Cease Concentration on Spell count as 0 FreeSwift Actions count as 1 action / Pro or Rea, but you always play the action before opponent.Immediate Actions count as 1 action / Pro or Rea, but you always play the action before opponent.Take 5-Foot Step Why do we need again this akward thing ?… – dropped !

Surprise!
When you are surprised you are not allowed to take any but Reflexes action (Saving Throw).

I hope most of you will catch the gist of this system and how it brings streamlining-simplicity in the combat sequence. It is the result what I expect to get IMO. Also it should be quite compatible with all the stuff but some feats and the attack of opportunity. Plus, I bet that it allows a good not boring and not stupid game.

C - Tracking & Loging of the actions
The mix of Initiative & Acting presented before is useful to track the rounds and the actions in a very simple way. It will be even better if you prepare the initiative chart(s) before the game. It is now possible because we don’t have to bother with Initiative roll and delayed / readied action.
Your chart should have 4 main columns.

is for Player and Monster ID

is for Player and Monster Initiative

is for Max number of actions (usually 2) / Max number of reactive (usually 1)

is for the action count with these symbols:

/ = no action (when surprised)

! = proactive

? = reactive

Repeat column D for each subsequent round.

D - Armor Class evolution
I move forward a little bit more my crusade for Simplicity in violating a sacred D&D taboo: … the AC.
I propose an alternative rule which streamlined the game but without expecting to reduce dice roll or (basic) calculation involved in the Hit process.
I explain first, and IMHO, how to reconsider AC in its two basic components so that the game becomes more instinctive. The AC is a mix of active and passive defence. It is sounded that the passive defence should be subtracted to the damages and not to the hit. About the active defence we can imagine it like a manoeuvre to be safe from an attempt to hit you. So this one has to act against the hit chance, and more, it is a saving throw type of thing.

The new AC (nAC) definition
It is a saving throw that its is not rolled and so valued at 10. To that 10 are added:

actual / current Reflexes Save modifier

modifier from class abilities, feats, skills and magical effect which are applying only on nCA

modifier from shield or spell-shield effect

Note:all the spells, effects, modifiers which apply to Reflexes will also apply on nAC by design. It is worth to recall a nAC is a Saving-Throw type thing here. The limits (in DEX) which can affect Reflexes Save because of the armour will also be detrimental to the nAC. Each enhancement to a shield is a direct enhancement to nAC. As usual bonuses coming from a same type of effect don’t stack.

The new DR (nDR) definition
To take care of the passive defence we define the nDR – please note that nDR is not supposed to be compatible with the actual DR / …. of the 3.5 Ed.
The nDR is subtracted to any damage but the continuous damages and the falling damages. It is:

brought by a vestment or a protection – or a spell like – we call an armour nDR

brought by a natural toughness – or a spell like – of the creature hide we call a natural armour nDR

Note: All the enhancement to an armour give a 1 for 1 nDR bonus. For conversion purpose and compatibility with weapon damages it is worth to calculate the nDR of any armour like halve (rounded up) of the AC bonus given by 3.5 Ed. – I fully catch that it means we have always two armours with same nDR but I think the differences in between armours can also be implemented thanks to other factors than nDR.
A very resistant material like adamantine add a flat +2 to the nDR of an armour. You should halve the natural armour AC modifier calculated like we do for armour.
As usual, bonuses coming from a same type of effect don’t stack.

Conversion and Compatibility
Compatibility with the 3.5 can only come from a conversion. Luckily is quite a basic process to do. The main stream is:

What about the 3.5 Ed DR ?
If it is a DR / - then you can definitely add it straight to the nDR. For those DR with / something, I guess it can be addressed like the invulnerability to all but something. It is a basic rule but it has a lot of flavour to make some enemies very frightful. I do prefer having a lycanthrope immune to all but silver for example.
If you are not pleased with this you can either keep the DR rule of 3.5 but it will add a lot of complexity with quite few advantages IMO (like it does actually).

Side note: A very important rule to implement with the nDR is that the critical roll always goes through it. It gives a streamlined and natural rule for critical hit. You have no longer to confirm your critical. If it’s a critical but not a hit then you don't take care of the nDR and deal with regular damages, if it is a critical and a hit then you apply the critical effect on damage and don't consider the nDR.